1. Field of the Invention
The present invention relates to a process for production of an ink jet head for forming ink droplets in an ink jet printing system.
2. Related Background Art
An ink jet head for an ink jet printing system (liquid discharge recording system) generally comprises a fine discharge opening (hereinafter occasionally referred to as an "orifice"), a liquid flow path communicating to the discharge opening, and an energy-generating portion positioned in the liquid flow path.
Such an ink jet head can be produced, for example, through the steps comprising: forming a photosensitive resin layer 2 on a substrate 1 on which a liquid discharge energy-generating portion 3 has been provided (FIG. 1A); patterning, by photolithography, the photosensitive resin layer 2 as a pattern 4 (hereinafter referred to as a "solid layer") for forming a liquid flow path on the substrate (FIG. 1B and FIG. 1C); covering the solid layer on the substrate with a liquid flow path wall construction material 5 composed of a curable resin, and curing this material 5 (FIG. 1D); and removing the aforementioned solid layer by dissolution by use of an organic solvent such as halogen-containing hydrocarbons, ketones, esters, and alcohols, or an aqueous alkali solution of sodium hydroxide or potassium hydroxide to form a liquid flow path 6 (FIG. 1E).
In recent years, with the demand for higher image quality, a larger number of the liquid flow paths are provided in a high density in the ink head. With the increased liquid flow path density, a liquid chamber for feeding the ink to the liquid flow paths should have a sufficient volume to supply the ink to the liquid flow paths at a high rate. To secure the sufficient volume of the liquid chamber, the aforementioned liquid flow path wall is formed higher. Conventionally, the liquid flow path wall is formed from a photosetting epoxy resin composition because of its high rigidity after curing, and its relatively high adhesiveness. However, for the larger thickness of the liquid flow path wall construction material as mentioned above, the light exposure conditions for the photosetting are severer, and may occasionally cause insufficient cure of the resin owing to insufficiency of light exposure.
Therefore, the inventors of the present invention tried to change the material for the liquid flow path construction from the conventional photosetting type epoxy resin composition to a thermosetting type one. A curing agent for thermosetting of an epoxy resin includes acid anhydride type and amine type of curing agents. On the other hand, the aforementioned solid layer is usually formed from a positive type resist containing naphthoquinone diazide. This type of resist is less heat-resistant, and will be deformed when the epoxy resin for the liquid flow path wall construction is cured with a curing agent such as an acid anhydride or an aromatic amine which requires a higher curing temperature of from 130 to 150.degree. C. Accordingly, as the curing agent for the epoxy resin, a cold cure type of amine curing agent is used which can cause cure at a relatively low temperature. However, the epoxy resin composition cured by cold cure to form the liquid flow path wall has generally a relatively low chemical resistance in comparison with the one formed by hot cure, and tends to swell greatly in the step of removal of the solid layer by a polar solvent (organic solvent) such as ketones, alcohols, and esters as a solid layer-removing liquid. The swelling of the liquid flow path wall construction material lengthens the time for removal of the solid layer, and lowers the productivity of the ink jet head.
Generally, the epoxy resin composition has high elasticity, which causes accumulation of high stress between the liquid flow path wall construction material and the substrate. The swelling of the liquid flow path wall construction material increases further the above stress to cause, in an extreme case, separation of the liquid flow path wall construction material from the substrate. In particular, in ink jet heads having liquid flow paths in a high density, the swelling becomes remarkable owing to fineness of the liquid flow path and the resulting long time of dissolution removal of the solid layer.
The requirement for the ink performance has become severer lately because of demands for higher image quality recorded by an ink jet printing system. For improvement of the ink, urea is incorporated in the ink as a humectant. Further, for improvement of water-resistance of the recorded matter, a slightly water-soluble dye is used as a coloring agent, and an alkali salt is incorporated in the ink to dissolve the dye. The ink containing urea or an alkali salt is relatively highly alkaline, which tends to swell the liquid flow path wall construction material similarly as the polar solvent. Accordingly, the conventional ink jet head produced by the above-mentioned process is liable to have the liquid flow path deformed by swelling due to the highly alkaline ink and thus to result in unsatisfactory liquid discharge, or is liable to cause separation of the liquid flow path wall construction material from the substrate, thus being not satisfactory in reliability of the ink jet head. As described above, the conventional production process for the ink jet head involves the technical problem of the swelling of the liquid flow path wall construction material by a polar solvent for removal of the solid layer or by a highly alkaline ink.
After comprehensive investigation of the thermosetting type epoxy resin composition as the liquid flow path wall construction material of the ink jet head, it was found by the inventors of the present invention that the above problem can be solved by use of a specific curing agent and a specific catalyst for the epoxy resin base material.